1. Field of the Invention
The present invention relates to a telephoto lens system and, more particularly, to a telephoto lens system allowing a short-distance photographing operation from infinity to an equal magnification.
2. Related Background Art
Conventionally, in order to perform a close-up photographing operation up to a magnification near an equal magnification, a system for increasing the extension amount of a photographing lens using a close-up member such as bellows is adopted, and such a photographing system is known to those who are skilled in the art. However, this system requires a cumbersome assembling operation, and is inconvenient for carrying. In addition, the extension amount becomes considerably large, resulting in insufficient performance. With recent spread of AF cameras comprising auto-focusing devices, demand has arisen for a decrease in work load as a product of the weight and the moving amount of a lens group, which is moved to attain an in-focus state. In order to quickly perform a focusing operation, and to prevent deterioration of image formation performance caused by the focusing operation in a short-distance photographing operation, a so-called telephoto macro lens adopting an internal focusing system or a floating system is proposed in, e.g., U.S. Pat. No. 4,437,734, Japanese Laid-Open Patent Application No. 2-81014, U.S. Pat. No. 4,508,433, Japanese Laid-Open Patent Application No. 61-132916, and the like.
In a telephoto lens system disclosed in U.S. Pat. No. 4,437,734 assigned to the owner of the present invention, since a first lens group at the most object side is fixed in position, advantages, e.g., no change in total length, and very small movement of the center of gravity can be expected. In this telephoto lens system, however, the focal length in an infinity arrangement is as relatively short as 162 mm or less, and a variation in astigmatism in an in-focus state is large. In addition, although this telephoto lens system realizes a photographing range from infinity to an equal magnification, a considerable aberration is generated near a 1/2 magnification (middle focal length range).
In a telephoto lens system disclosed in Japanese Laid-Open Patent Application No. 2-81014 assigned to the owner of the present invention, most of the problems of the above-mentioned patent are solved, but a problem of a large distortion in an equal magnification photographing operation remains unsolved. In a telephoto lens system disclosed in U.S. Pat. No. 4,508,433, the focal length in the infinity arrangement is as short as about 100 mm, and a change in total length and a work load in a focusing operation are large. Furthermore, a problem of a large variation in spherical aberration remains unsolved.
A telephoto lens system disclosed in Japanese Laid-Open Patent Application No. 61-132916 has a short total length to the focal plane and a focal length of about 200 mm in the infinity arrangement, and causes a relatively small variation in aberration. However, since this telephoto lens system adopts a focusing system for extending a first lens group at the most object side, which is constituted by a large number of lenses and is heavy, it suffers from a large change in total length, a large movement of the center of gravity, and a large work load in a focusing operation from infinity to a short distance.